The present invention relates generally to voltage controlled oscillators and more particularly to a narrow band voltage controlled oscillator which has a designed center frequency about which the output frequency can be varied in response to an input control signal.
There are, of course, a large number of different types of voltage controlled oscillators (VCO) which are known in the art and no attempt is here made to give a summary of these many types. In the general area of VCOs using operational amplifiers and multipliers, however, reference is made to the Burr-Brown text Operational Amplifiers published by McGraw-Hill Book Company (Copyright 1971--Library of Congress Catalog Card Number 74-163297). On page 402 of that publication there is shown a circuit which employs multipliers and operational amplifiers connected in an integration mode to provide a broad band VCO which produces both the direct and quadrature components of the oscillator output in response to a variable input signal. Although this teaching is, apparently, for normal broad band applications, (e.g., from zero to some finite output frequency) it is conceivable that an offset to some center frequency could be achieved by the inclusion of some form of biasing. Such biasing, which usually takes the form of a bias voltage, tends to have operational problems, however, since it is very difficult (or at least very expensive) to provide a bias power supply which does not drift.
In the aforementioned U.S. Pat. No. 3,978,420 there is described a self-tuning filter which has excellent stability characteristics and which is comprised, primarily, of multipliers and integrators with appropriate feedback paths. In the previously referenced application Ser. No. 383,076 there is described a frequency to voltage transducer which employs a modified version of the patent teaching.